Tracking control of radiation flying spot scanning systems is desirable in the wire transmission of picture material, in television scanning apparatus and, particularly, in the close tolerance inspection of rapidly moving product webs. This invention is hereinafter described relative to flying spot web inspection, where it has become more and more difficult to inspect relatively wide webs, due to increasing manufacturing speeds, with scanning beams having small cross sections, e.g., on the order of 1 mm dia., to detect small pin hole type defects without danger of gaps occurring in the scan coverage of the inspected product and, of course, without spurious signal generation.
Of particular concern is the attention given to the mechanical and optical design of scanning systems, especially with respect to the effects of vibration, component misalignment, component manufacturing precision, scanner motor-bearing play, temperature changes and the like, which all have an effect on the tracking of a scanning radiation beam.
For example, multifaceted rotating mirror cone angle variations, i.e., the angle which each facet makes with the scanner axis of rotation, usually varying between .+-.0.05 m rad and .+-.0.15 m rad, cause corresponding scan path displacements. In addition, temperature changes can introduce still other variations in rotating mirror cone angles, or further distort the inspection system support framework to affect optical alignment.
The prior art teaches solutions of some of the adverse problems bearing on the constancy of scan-to-scan spacing, especially as regards information recording using light beams. However, no one has devised a way to compensate for these influences effectively at extremely high scanning rates. The usual technique has included the use of beam splitters to form an auxiliary monitoring beam from the main scanning beam, as taught in IBM Technical Disclosure Bulletin, Vol. 15, No. 1, June, 1972, "Deflection Sensor for Optical Scanners" by D. H. Casler, D. R. Cecchi and W. D. McNeil; U.S. Pat. No. 3,715,599 issued Feb. 6, 1973 to R. Marcy, entitled "Electro-Optical System for Controlling the Attitude of a Stage Mounted on a Carriage Sliding Along a Parallel Bench"; and U.S. Pat. No. 3,646,568 issued Feb. 29, 1972, entitled "Beam Control System". To avoid the additional optical losses introduced by the use of beam splitters in a flying spot inspection system, an alternate accepted practice has been to apply a programmed set of predetermined control voltages to the beam deflector to correct for each rotating mirror facet's known variation from a norm, as taught in U.S. Pat. No. 3,809,806 issued May 7, 1974 to Walker et al., entitled "Banding Correction System for Film Recording Apparatus", and the article entitled "Correction of Axial Deflection Errors in Rotating Mirror Systems" by J. Helmberger et al., Optics and Laser Technology, Dec. 1975, pp. 249-252.